In the conventional art, compact disks (herein after referred to as CDs) whereon audio information or other information is recorded in the form of digital signals through minute pits that can be detected optically, are widely used as optical disks. The information recorded on a CD is reproduced by means of a Read-Only optical disk recording/reproducing device.
In a CD, a plurality of pieces of information are usually recorded in succession. In addition, absolute addresses that provide information concerning recording/reproducing positions on the disk are pre-recorded on the disk in the form of physical alterations. During the reproduction, absolute addresses read from the disk and absolute addresses indicating the recording start position of each piece of information and recorded in a TOC (Table Of Contents) area provided in the inner periphery or other location of the disk, are compared, and the desired pieces of information may be reproduced successively or selectively.
An optical disk recording/reproducing device that records and reproduces audio information or other information on rewritable optical disks developed recently such as magneto-optical disks or Write-Once type optical disks where information can be recorded only once, should preferably adopt the same method of reproduction as the one used in a conventional optical disk recording/reproducing device for. CDs only and should be compatible with the different types of optical disks in use. In this case, a Constant Linear Velocity (hereinafter referred to CLV) that is employed with CDs may be adopted as rotation control method for the writable or write-once optical disk during recording and reproduction.
The CLV method may be implemented by, for example, controlling the rotational speed of a motor in accordance with the radial position of an optical head. In this case, the rotational speed of the motor is detected by means of a rotary encoder or other member, and the radial position of the optical head is detected by means of a position detecting sensor. However, when the CLV method is implemented in such a manner, rotation control that is sufficiently accurate cannot be expected due to errors in, the detections performed by the rotary encoder and the position detecting sensor.
The accuracy of the rotation control may be improved by enforcing the CLV method according to disk position information that was pre-recorded on the optical disk such as the absolute addresses described above. In the conventional art, Japanese Publication for Unexamined Patent Application 1989-39632 (Tokukaisho 64-39632) discloses a method for recording absolute addresses. Namely, the disk position information goes through a Biphasemark modulation process and the guiding groove of the optical disk is made to deviate inward or outward in a radial direction or the width of the guiding groove is changed, depending on whether the modulated code is "1" or "0". The absolute adresses and the recorded information can be reproduced separately by having their respective frequency bandwidths differ from each other.
However, even if the information is recorded while the optical disk is driven to rotate according to the information pre-recorded on the optical disk such as absolute addresses, etc., in practice, there is a possibility that the information might be recorded in a condition that is slightly different from the CLV requirements because of CLV control errors occurring during recording. In this case, if the information recorded is audio information, variations might occur in the speed at which music is played during the reproduction. Therefore, during the reproduction, a CLV control that is based on the information to be reproduced should be performed.
However, reproduction, errors occur frequently, and when the optical head moves from a recorded area to an unrecorded area, rotation control based on the reproduced information becomes infeasible. In addition, once the rotation control bases on the reproduction information has become infeasible, a significant lapse of time is needed before the rotation control can be executed correctly even after the quality of the reproduced signals has been restored.